Crack Propagation Characteristics in Three Deep Hardenable Titanium Alloys.

摘要

Tests were conducted on three recently developed deep-hardenable grade titanium alloys in order to assess their crack propagation and stress corrosion behavior. The alloy compositions are as follows: Alloy no. 334 (Ti-10Mo-6Cr-2.5Al); alloy no. 227 (Ti-7Mo-4Cr-2.5Al); and Alloy no. 253 (Ti-10Mo-8V-2.5Al). Compact tension specimens were cut from 6-inch diameter forging billets in three orientations. Each orientation was subsequently tested for fatigue crack growth rate and stress corrosion susceptibility. While all three alloys exhibited similar fatigue crack propagation characteristics, the chromium containing alloys appeared to be more susceptible to stress corrosion cracking. Scanning electron microscopy of the fracture surfaces revealed the presence of an apparently brittle rod-like phase. While the composition of the rods could not be directly determined, indirect evidence indicated that the rods were titanium borides. These rods were judged to have a significant effect on the fracture toughness and a lesser effect on the crack propagation properties of the alloys.